EP1879291A1 - Rand-schallwelleneinrichtung - Google Patents

Rand-schallwelleneinrichtung Download PDF

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Publication number
EP1879291A1
EP1879291A1 EP06713834A EP06713834A EP1879291A1 EP 1879291 A1 EP1879291 A1 EP 1879291A1 EP 06713834 A EP06713834 A EP 06713834A EP 06713834 A EP06713834 A EP 06713834A EP 1879291 A1 EP1879291 A1 EP 1879291A1
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Prior art keywords
medium
idt
boundary
waves
boundary acoustic
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French (fr)
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EP1879291A4 (de
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Hajime c/o MURATA MANUFACTURING CO. LTD. KANDO
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Murata Manufacturing Co Ltd
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Murata Manufacturing Co Ltd
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    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03H—IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
    • H03H9/00—Networks comprising electromechanical or electro-acoustic devices; Electromechanical resonators
    • H03H9/02—Details
    • H03H9/0222—Details of interface-acoustic, boundary, pseudo-acoustic or Stonely wave devices
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03H—IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
    • H03H9/00—Networks comprising electromechanical or electro-acoustic devices; Electromechanical resonators
    • H03H9/02—Details
    • H03H9/02535—Details of surface acoustic wave devices
    • H03H9/02543—Characteristics of substrate, e.g. cutting angles
    • H03H9/02559—Characteristics of substrate, e.g. cutting angles of lithium niobate or lithium-tantalate substrates
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03H—IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
    • H03H9/00—Networks comprising electromechanical or electro-acoustic devices; Electromechanical resonators
    • H03H9/02—Details
    • H03H9/125—Driving means, e.g. electrodes, coils
    • H03H9/145—Driving means, e.g. electrodes, coils for networks using surface acoustic waves
    • H03H9/14538—Formation

Definitions

  • the present invention relates to boundary acoustic wave devices using boundary acoustic waves propagating along the interface between different media, and more particularly to a boundary acoustic wave device including a multilayer structure formed by stacking at least three media.
  • Boundary acoustic waves propagate along the interface between different media. Therefore the packages of boundary acoustic wave devices can be simpler than those of surface acoustic wave devices using surface acoustic waves. The boundary acoustic wave device thus can be more simplified, and its thickness can be reduced.
  • Non-Patent Document 1 has disclosed a boundary acoustic wave device.
  • the boundary acoustic wave device has a multilayer structure including a first medium of SiO 2 or Si, a ZnO third medium and a SiO 2 second medium stacked in that order.
  • An IDT interdigital transducer
  • the vibrational energy of boundary acoustic waves is confined in the third medium made of ZnO in which acoustic velocity becomes low, and thus boundary acoustic waves are propagated.
  • the IDT is made of A1.
  • Patent Document 1 has disclosed a boundary acoustic wave having a multilayer structure including a first medium, a third medium and a second medium stacked in that order as in Non-Patent Document 1.
  • the first medium is made of LiNbO 3
  • the third medium is made of SiO 2
  • the second medium is made of SiN.
  • An Al IDT is disposed between the first medium and the third medium.
  • Non-Patent Document 1 IEICE material, 1986, S86-39, pp. 47-4 .
  • Patent Document 1 WO98/52279
  • the boundary acoustic wave devices disclosed in Non-Patent Document 1 and Patent Document 1 each have an Al IDT.
  • boundary acoustic wave devices using A1 electrodes the acoustic velocity of transverse waves tends to be higher, and the confinement efficiency of the vibrational energy of the boundary acoustic waves tends to be lower, in comparison with boundary acoustic wave devices using electrodes made of a metal having a higher density than Al, such as Au, Ag, or Cu.
  • the confinement of the vibrational energy mainly depends on the third medium in which the acoustic velocity of transverse waves is low, and nobody has thought that confinement of the vibrational energy can be achieved by appropriately selecting the material of electrodes. Accordingly, the confinement efficiency of the vibrational energy is not satisfactory, and the thicknesses of the first and second media are increased. It has been thus considered that boundary acoustic wave devices are difficult to downsize.
  • SiO 2 While many of the materials used as the first to third media propagating boundary acoustic waves have negative temperature coefficients of acoustic velocity (TCV), SiO 2 has a positive TCV. Hence, a combination of SiO 2 and a material having a negative TCV can make the TCV value zero or close to zero.
  • TCV acoustic velocity
  • the frequency temperature coefficient TCF of the boundary acoustic wave device results from the subtraction of the linear expansion coefficient of the boundary wave propagation path from the TCV.
  • a combination of SiO 2 and another medium material can achieve a boundary acoustic wave device having a low frequency temperature coefficient TCF.
  • the IDT of such a known boundary acoustic wave device is made of Al, as described in Non-Patent Document 1 and Patent Document 1.
  • the SiO 2 fills the spaces between the Al strips arranged at periodic intervals of the IDT and the reflectors.
  • the difference in density between Al and SiO 2 is small, and the difference in acoustic impedance between them is also small. Accordingly, the reflection of the boundary acoustic waves from the IDT and reflectors is reduced for each A1 strip.
  • the reflectors are inevitably formed large, and the resulting boundary acoustic wave device must be large.
  • the shape factor of a longitudinally coupled resonator-type boundary acoustic wave filter or the directivity of the EWC SPUDT of a transversal boundary acoustic wave filter is degraded, for example.
  • boundary acoustic waves propagate with the vibrational energy confined in the third medium and the IDT. If the thickness of the third medium is relatively large with the wavelength of propagating boundary waves, higher-order modes are relatively strongly excited. Therefore the thickness of the third medium is preferably smaller than or equal to the wavelength of a single wave of the boundary acoustic waves.
  • the third medium is formed by deposition, such as sputtering, it is difficult to increase the thickness of the third medium to a sufficiently larger value than the thickness of the strips of the IDT and reflectors.
  • a third medium having a small thickness may be cracked due to the unevenness between regions having the strips and regions having no strips.
  • an object of the present invention is to provide a boundary acoustic wave device that has a multilayer structure including a first medium, a third medium and a second medium stacked in that order, that can efficiently confine the vibrational energy of boundary acoustic waves in the third medium so as to exhibit a low boundary acoustic wave propagation loss and a high electromechanical coupling coefficient so as not to be affected by higher-order modes to produce undesired spurious responses, and thus can produce superior resonance characteristics or filter properties, and that is not easily cracked in the third medium.
  • a boundary acoustic wave device which has a multilayer structure including a first medium having piezoelectric characteristics, a non-electroconductive second medium, and a third medium through which slow transverse waves propagate at a lower acoustic velocity than slow transverse waves propagating through the first and second media.
  • the first medium, the third medium, and the second medium are stacked in that order.
  • An IDT disposed between the first medium and the third medium.
  • the IDT includes a metal layer made of a metal having a density ⁇ in the range of 3000 to 21500 kg/m 3 .
  • the IDT has electrode fingers at a pitch of ⁇ and has a thickness H1 satisfying the relationship 0.006 ⁇ ⁇ H1 ⁇ 0.2 ⁇ , and the third medium has a thickness H2 satisfying the relationship H1 ⁇ H2 ⁇ 0.7 ⁇ .
  • the third medium has a thickness H2 satisfying the relationship H1 ⁇ H2 ⁇ 0.5 ⁇ .
  • the third medium is made of SiO 2 or a material mainly containing SiO 2 .
  • the first medium is made of LiNbO 3 and has Euler angles [ ⁇ , ⁇ , ⁇ ] satisfying the relationships -25° ⁇ ⁇ ⁇ 25°, 92° ⁇ ⁇ ⁇ 114°, and -60° ⁇ ⁇ ⁇ 60°.
  • the first medium is made of LiNbO 3 and has Euler angles [ ⁇ , ⁇ , ⁇ ] satisfying the relationships -25° ⁇ ⁇ 25°, 92° ⁇ ⁇ ⁇ 114°, and 60° ⁇ ⁇ ⁇ 120°.
  • the first medium is made of LiNbO 3 and has Euler angles [ ⁇ , ⁇ , ⁇ ] satisfying -32° ⁇ ⁇ ⁇ 32°, 15° ⁇ ⁇ ⁇ 41°, and -35° ⁇ ⁇ ⁇ 35°.
  • the IDT is made of a metal selected from the group consisting of Pt, Au, Cu, Ag, Ni, Ti, Fe, W, Ta, and alloys mainly containing those metals.
  • the IDT has a structure formed by alternately disposing a first metal layer having a relatively high density and a second metal layer having a relatively low density.
  • the first metal layer is disposed at the first medium side.
  • the first medium and/or the second medium has a multilayer structure including a plurality of medium layers.
  • the boundary acoustic wave device of the present invention includes a multilayer structure and an IDT.
  • the multilayer structure includes a first medium having piezoelectric characteristics, a non-electroconductive second medium, and a third medium through which slow transverse waves propagate at a lower acoustic velocity than slow transverse waves propagating through the first and second media.
  • the first medium, the third medium and the second medium are stacked in that order to define the multilayer structure.
  • the IDT is disposed between the first medium and the third medium. Since the IDT is made of a metal having a density ⁇ in the range of 3000 to 21500 kg/m 3 , the propagation loss of boundary acoustic waves can be reduced, and the loss in the boundary acoustic wave device can be reduced.
  • the thickness H1 of the IDT is in the range of 0.006 ⁇ ⁇ H1 ⁇ 0.2 ⁇ , the electromechanical coupling coefficient is sufficient for boundary acoustic waves. Also, the difference in acoustic velocity between SH boundary waves and P+SV boundary waves can be increased. When SH boundary waves are used in the main mode, spuriouses of P+SV boundary waves, which are unnecessary to the SH boundary waves, can be reduced. In addition, since the thickness H2 of the third medium is in the range of H1 ⁇ H2 ⁇ 0.7 ⁇ , higher-order spuriouses of SH boundary waves can also be reduced.
  • the present invention can provide a low-loss boundary acoustic wave device having such a high electromechanical coupling coefficient K 2 that the size, particularly thickness, can be reduced, and exhibiting superior resonance characteristics or filter properties.
  • the third medium may be made of SiO 2 or a SiO 2 -based material and the SiO 2 has a positive TCV.
  • many of the materials of the media of boundary acoustic wave devices have negative TCV's.
  • the third medium made of SiO 2 or a SiO 2 -based material has a low frequency temperature coefficient TCF and the resulting boundary acoustic wave device can exhibit superior temperature characteristics.
  • the electromechanical coupling coefficient K 2 for P+SV boundary waves can be sufficiently reduced, so that SH boundary waves can be used to achieve superior resonance characteristics or filter properties.
  • the electromechanical coupling coefficient K 2 for SH boundary waves can be sufficiently recued, so that P+SV boundary waves can be used to achieve superior resonance characteristics or filter properties.
  • At least one of the second metal layers contains Al, which has a low electrical resistance, the electrical resistance loss due to the electrode fingers can be reduced more effectively.
  • the electromechanical coupling coefficient K 2 for SH boundary waves can be sufficiently reduced, so that P+SV boundary waves can be used to achieve superior resonance characteristic or filter properties.
  • the IDT can be made of a metal selected from the group consisting of Pt, Au, Cu, Ag, Ni, Ti, Fe, W, Ta, and alloys mainly containing those metals. Since those metals have higher densities than Al, the propagation loss of boundary acoustic waves can be reduced and thus a low-loss boundary acoustic wave device can be achieved.
  • the IDT may be defined by a single metal layer, or a multilayer structure including a first metal layer having a relatively high density and a second metal layer having a relatively low density. If the multilayer structure is formed by alternately disposing the first metal layers and the second metal layers, the thickness of the electrode fingers can be increased on the condition that the propagation loss of boundary acoustic waves is reduced. Thus, the electrical resistance loss due to the electrode fingers can be reduced.
  • the first metal layer may be disposed at the first medium side.
  • the first metal layer having a relatively high density is disposed at the side of the first medium having a low acoustic velocity. Accordingly, a larger amount of energy of boundary acoustic waves is distributed at the first medium side.
  • the first medium is made of a piezoelectric material, the electromechanical coupling coefficient K 2 can be increased.
  • the first medium and/or the second medium have a multilayer structure defined by a plurality of medium layers including, for example, a compressive stress layer and a tensile stress layer, the total stress can be reduced by the interaction of these stresses.
  • the frequency can be controlled by adjusting the thickness of either the first or the second medium by etching, such as ion beam etching. If a plurality of layers each have a sufficiently lower thickness than ⁇ , the multilayer structure can produce an intermediate acoustic velocity for the layers of the multilayer structure.
  • Euler angles the crystallographic axis, and equivalent Euler angles refer to the following.
  • the Euler angles ( ⁇ , ⁇ , ⁇ ) representing the direction of the cut surface of the substrate and the propagation direction of the boundary waves are based on the right-handed Euler angles described in a document "Danseiha Soshi Gijutsu Handbook (Acoustic Wave Device Technology Handbook)" (Japan Society for the Promotion of Science, Acoustic Wave Device Technology the 150th Committee, 1st Version 1st printing, published on January 30, 2001, p. 549). Specifically, in LN crystallographic axes X, Y, and Z, the X axis is rotated ⁇ turn anticlockwise about the Z axis to define a Xa axis.
  • the Z axis is rotated ⁇ turn anticlockwise about the Xa axis to define a Z' axis.
  • a plane including the Xa axis whose normal is the Z' axis is defined as the cut surface.
  • the propagation direction of the boundary waves is set to be the direction of the X' axis that is defined by rotating the Xa axis ⁇ turn anticlockwise about the Z' axis.
  • the Z axis is set to be parallel to the c axis
  • the X axis is set to be parallel to one of the equivalent a axes extending in three directions
  • the Y axis is set to be the normal of a plane including the X axis and the Z axis.
  • the present invention requires only that the LiNbO 3 Euler angles ( ⁇ , ⁇ , ⁇ ) be crystallographically equivalent.
  • a document Nihon Onkyo Gakkai-shi (Journal of the Acoustical Society of Japan) Vol. 36, No. 3, 1980, pp. 140-145 ) has taught that LiNbO 3 belongs to the trigonal 3m point group, and Equation [4] therefore holds.
  • F represents a boundary wave property, such as electromechanical coupling coefficient k s 2 , propagation loss, TCF, PFA, or a natural unidirectional property.
  • k s 2 propagation loss
  • TCF propagation loss
  • PFA propagation loss
  • TCF propagation loss
  • PFA propagation loss
  • the propagation characteristics of boundary waves with Euler angles of (30°, ⁇ , ⁇ ) are equivalent to those of boundary waves with Euler angles of (90°, 180°- ⁇ , 180°- ⁇ ).
  • the propagation characteristics of boundary waves with Euler angles (30°, 90°, 45°) are equivalent to those of boundary waves with Euler angles shown in Table 1.
  • the constants of conductive material used for the calculations in the present invention are those of polycrystals, epitaxial films or the like can also produce boundary wave propagation characteristics to the extent that problems do not occur in practice according to Equation [4] because the crystal orientation dependence of the substrate is more dominant to the boundary wave characteristics than the crystal orientation dependence of the layers.
  • Fig. 1 is a schematic sectional plan view showing the structure of the electrodes of a boundary acoustic wave device according to an embodiment of the present invention
  • Fig. 2 is a sectional front view of the boundary acoustic wave device.
  • the boundary acoustic wave device 10 of the present embodiment includes a third medium 13 and a second medium 12 formed in that order on a first medium 11 having piezoelectric characteristics.
  • An IDT 14 and reflectors 15 and 16 are disposed along the interface between the first medium 11 and the third medium 13. In other words, electrodes are disposed along the interface between the first and third media 11 and 13.
  • the IDT 14 has a plurality of electrode fingers 14a and a plurality of electrode fingers 14b that are alternately disposed between other electrode fingers.
  • the electrode fingers 14a are electrically connected to one bus bar, and the other electrode fingers 14b are electrically connected to the other bus bar.
  • the IDT 14 is weighted by varying the finger overlap.
  • the reflectors 15 and 16 are disposed outside the direction perpendicular to the direction in which the fingers 14a and 14b of the IDT 14 extend, that is, at both sides of the direction at which boundary acoustic waves propagate.
  • the IDTs 15 and 16 each have a plurality of electrode fingers extending in the direction perpendicular to the direction in which the boundary acoustic waves propagate, and the ends of these electrode fingers are closed together. While the ends of the reflectors are closed together in the embodiment, OPEN reflectors having open ends may be used.
  • the third medium 13 in which slow transverse waves used in the device have a relatively low acoustic velocity is disposed between the first and second media 11 and 12 in which the slow transverse waves have relatively high acoustic velocities. Consequently, boundary acoustic waves are propagated while their vibrational energy is confined in the third medium 13 exhibiting a relatively low acoustic velocity. More specifically, boundary acoustic waves are propagated in the direction perpendicular to the electrode fingers 14a and 14b and parallel to the plane on which the IDT 14 is formed, by confining the vibrational energy of the boundary acoustic waves between the interface of the second and third media 12 and 13 and the interface of the first and third media 11 and 13.
  • the first medium 11 is made of piezoelectric 15° Y-cut X-propagating LiNbO 3 having Euler angles of (0°, 105°, 0°).
  • the second medium 12 is made of non-electroconductive SiN.
  • the third medium 13 is made of SiO 2 .
  • the IDT 14 and the reflectors 15 and 16 are made of a metal having a higher density than A1. More specifically, the IDT 14 is made of a metal having a density ⁇ in the range of 3000 to 21500 kg/m 3 .
  • the IDT 14 has a thickness H1 in the range of 0.006 ⁇ ⁇ H1 ⁇ 0.2 ⁇ and the third medium 13 has a thickness H2 in the range of H1 ⁇ H2 ⁇ 0.7 ⁇ , where ⁇ represents the pitch of electrode fingers of the IDT 14.
  • the boundary acoustic wave device 10 can exhibit low loss characteristics and can be downsized.
  • the boundary acoustic wave device 10 also has a high electromechanical coupling coefficient K 2 for boundary acoustic waves and is accordingly not affected by higher-order spurious responses, thus exhibiting superior characteristics. This is further described with reference to specific experiments.
  • the electrodes of the boundary acoustic wave device were evaluated.
  • the electrodes were formed of Al, which is conventionally used as the electrode material of the IDT, or Cu, Ag or Au that has a higher density than A1. The results are shown in Figs. 3 to 18.
  • the thickness of the third medium or the SiO 2 layer was set at 0.5 ⁇ , and the thicknesses of the first and second media were set infinite.
  • U2 shows the results for boundary acoustic waves essentially composed of SH waves
  • U3 shows the results for boundary acoustic waves essentially composed of P+SV components.
  • the coupling of SV+P boundary acoustic waves with the piezoelectric characteristics is weak. Accordingly, the SV+P boundary waves are hardly excited, and SH boundary waves are used as boundary acoustic waves in the main mode.
  • Equation (1) Vf represents the acoustic velocity at an open boundary, and V represents the acoustic velocity at a closed boundary.
  • K 2 2 ⁇ Vf - V / Vf
  • TCF ( V [ 30 ⁇ °C ] - V 20 ⁇ °C ) / V 25 ⁇ °C / 10 ⁇ °C - ⁇ s
  • ⁇ S represents the linear expansion coefficient of the first medium 11 in the direction in which boundary acoustic waves propagate.
  • the acoustic velocities of longitudinal waves, fast transverse waves and slow transverse waves in a Y-rotated X-propagating LiNbO 3 are 6547, 4752 and 4031 m/s, respectively.
  • the acoustic velocities of longitudinal waves and slow transverse waves in the SiO 2 layer are 5960 and 3757 m/s, respectively.
  • the acoustic velocities of longitudinal waves and slow transverse waves in the SiN layer are 10642 and 5973 m/s, respectively.
  • Figs. 3 to 6 and Figs. 11 to 14 show that in any case using different metal electrodes, the propagation loss ⁇ m of SH boundary waves is 0 at a thickness at which the acoustic velocity of SH boundary acoustic waves is less than or equal to the lowest value 4031 m/s of the acoustic velocities of longitudinal waves, fast transverse waves and slow transverse waves.
  • the acoustic velocity of boundary acoustic waves is 4031 m/s or less.
  • the thickness of A1 electrodes must be large.
  • electrodes made of Cu, Ag, or Au, which has a higher density than Al can lead to a propagation loss ⁇ m of 0 by using thinner IDT than the Al IDT.
  • a lager thickness of the electrodes increases the difference in height between the regions where the electrode fingers of the IDT 14 and reflectors 15 and 16 are present and the regions where the electrode fingers are not present. Accordingly, if the third medium 13 and the second medium 12 are formed by sputtering or evaporation in the portions of medium in which the IDT 14 and reflectors 15 and 16 are arranged, their coverage is not sufficient and the third medium 13 and the second medium 12 may be cracked. In addition, the deposition time is increased, and consequently the cost for forming the third medium 13 and the second medium 12 is liable to increase.
  • SH boundary waves as the main mode, therefore, spurious responses of the P+SV boundary waves acting as unnecessary modes can be reduced by forming the electrodes of a metal having a higher density than Al to a thickness in the range of 0.006 ⁇ to 0.2 ⁇ .
  • a conductive material having a relatively high density can produce a higher electromechanical coupling coefficient K 2 .
  • An IDT 14 made of a conductive material having a high density concentrates the vibrational energy in the vicinity of the IDT 14 because the acoustic velocity in the conductive material is very low.
  • the first medium 11 is formed of a piezoelectric material with the IDT 14 in contact with the piezoelectric material, the energy in the piezoelectric material is increased and thus the electromechanical coupling coefficient K 2 is further increased.
  • a design technique for adjusting the electromechanical coupling coefficient K 2 with propagation angle has been known. If the electromechanical coupling coefficient K 2 is increased, the range of its adjustment expands. Accordingly, the range of design can be further increased.
  • Pt electrodes resulted in substantially the same effect as the Au electrodes. Since Pt has a slightly higher density than Au, the vibrational energy tended to concentrate more in the vicinity of the IDT 14.
  • Figs. 49 to 51 show displacement distributions of SH boundary acoustic waves in use of an IDT 14 defined by a 0.05 ⁇ thick Al layer, a 0.10 ⁇ thick Al layer, or a 0.05 ⁇ thick Au layer.
  • the vibrational energy is emitted to the LiNbO 3 side or first medium side as is clear from Fig. 49, and hence boundary acoustic waves are not sufficiently confined in the boundaries between the media.
  • the vibrational energy is prevented from emitting, but is distributed in the LiNbO 3 layer of the first medium 11 up to a depth of 5 ⁇ or more (not shown in the figure) and in the SiN layer of the second medium up to a depth of about 1.2 ⁇ , as is clear from Fig. 50.
  • the results suggest that the thicknesses of the first and second media must be large.
  • the vibrational energy is distributed only in the region of the LiNbO 3 layer up to a depth of 0.9 ⁇ and the region of the SiN layer up to a depth of 0.9 ⁇ even though the IDT thickness is as small as 0.05 ⁇ .
  • the use of an IDT 14 made of Au having a higher density ⁇ than Al allows the reduction of the thicknesses of both the SiN second medium and the LiNbO 3 first medium. Consequently, the resulting boundary acoustic wave device can be thin and its manufacturing cost can be reduced.
  • SiO 2 has a density of 2210 kg/m 3 , and an acoustic characteristic impedance of 8.3 ⁇ 10 6 kg ⁇ s/m 2 for transverse waves
  • Al has a density of 2699 kg/m 3 , and an acoustic characteristic impedance of 8.4 ⁇ 10 6 kg ⁇ s/m 2 for transverse waves
  • Cu has a density of 8939 kg/m 3 , and an acoustic characteristic impedance of 21.4 ⁇ 10 6 kg ⁇ s/m 2 for transverse waves
  • Ag has a density of 10500 kg/m 3 , and an acoustic characteristic impedance of 18.6 ⁇ 10 6 kg ⁇ s/m 2 for transverse waves
  • Au has a density of 19300 kg/m 3 , and an acoustic characteristic impedance of 24.0 ⁇ 10 6 kg ⁇ s/m 2 for transverse waves.
  • the strips Since the differences in density and acoustic characteristic impedance between SiO 2 and Al are small, the strips have a low reflection coefficient in the structure including a SiO 2 third medium 13 and an Al IDT 14. Accordingly, a large number of strips are required in order to ensure a sufficient reflection coefficient of the reflectors 15 and 16. This disadvantageously increases the size of the device. If the reflection of the strips of the IDT 14 is reduced, the shape factor of a longitudinally coupled resonator-type boundary acoustic wave filter or the directivity of the EWC SPUDT of a transversal boundary acoustic wave filter is disadvantageously degraded.
  • the strips of the IDT 14 have a sufficiently high reflection coefficient in the structure including a SiO 2 third medium 13 and a Au IDT 14.
  • the reflectors 15 and 16 also have a sufficiently high reflection coefficient even if the number of strips is small. Consequently, a longitudinally coupled resonator-type filter having a superior shape factor, or a SPUDT having a high directivity can be achieved.
  • the third medium 13 has a certain thickness, and the interface between the second medium 12 and the third medium 13 and the interface between the third medium 13 and the first medium 11 are formed on and under the third medium 13. If the thickness of the third medium 13 is increased, a higher-order mode occurs in which waves propagates with confinement between those interfaces.
  • Figs. 19 and 20 are each a representation of the relationship between higher order modes and the thickness of the SiO 2 layer of a SiN/SiO 2 /IDT/LiNbO 3 multilayer structure including an Al IDT or Au IDT.
  • the LiNbO 3 layer had Euler angles of (0°, 105°, 0°) and the IDT had a thickness of 0.05 ⁇ .
  • Fig. 19 clearly shows that when the SiO 2 layer has a thickness of 0.9 ⁇ or less in the structure having the Al IDT, higher order SH boundary wave spuriouses are cut off.
  • the SiO 2 layer has a thickness of 0.7 ⁇ or less in the structure having the Au IDT, higher-order SH boundary waves can be cut off, as shown in Fig. 20.
  • the use of high-density Au electrodes allows the thickness of the SiO 2 layer or third medium 13, which can cut off higher mode waves, to be reduced. Accordingly, the boundary acoustic wave device can be downsized.
  • Fig. 20 clearly shows that a third medium 13 with a smaller thickness allows the acoustic velocity of the main-mode waves SHO to differ largely from that of higher-order waves designated by, for example, SH1.
  • the difference in frequency between the main mode and higher modes can advantageously be increased.
  • the third medium 13 In use of an IDT 14 made of Au or a metal having a higher density than A1, higher order modes can be cut off by setting the thickness of the third medium 13 to 0.7 ⁇ or less, as is clear from Fig. 20. Furthermore, in order to adapt to variations in manufacture or in order to cut off higher order modes from a wide range of frequency bands, the third medium 13 preferably has a thickness of 0.5 ⁇ or less.
  • the difference in response frequency between the main mode and higher order modes can be sufficiently increased by reducing the thickness of the third medium 13 to 0.7 ⁇ or less, preferably to 0.5 ⁇ or less.
  • Main mode P+SV boundary waves can be easily produced by using, for example, a 120° Y-cut X-propagating LiNbO 3 substrate having Euler angles of (0°,30°, 0°).
  • Figs. 21 to 24 show the relationships of ⁇ of the Euler angles (0°, ⁇ , 0°) of the LiNbO 3 first medium 11 with the acoustic velocity of boundary acoustic waves, the electromechanical coupling coefficient K 2 , the propagation loss, and the frequency temperature coefficient TCF, respectively.
  • U2 shows the results for boundary waves essentially composed of SH components
  • U3 shows the results for boundary waves essentially composed of P+SV components. The results were obtained under the following conditions.
  • Multilayer structure SiN/SiO 2 /IDT/LiNbO 3 , where the LiNbO 3 is the first medium 11; the SiN is the second medium 12; and the SiO 2 is the third medium 13.
  • the thickness of the SiN second medium 12 was infinite, the thickness of the first medium 11 was infinite, the thickness of the SiO 2 third medium 13 was 0.5 ⁇ ; and the IDT 14 was formed of Au to a thickness of 0.07 ⁇ .
  • Figs. 25 to 28 are representations of the relationships of ⁇ of the Euler angles (00, 104°, ⁇ ) with the acoustic velocity of boundary acoustic waves, the electromechanical coupling coefficient K 2 , the propagation loss, and the frequency temperature coefficient TCF, respectively, obtained in the same manner as Figs. 21 to 24.
  • the Euler angle ⁇ represents the direction in which boundary acoustic waves propagate on the substrate.
  • the electromechanical coupling coefficient for SH boundary waves can be adjusted in the range of 0.1% to 17.8% by setting the Euler angle ⁇ in the range of 0° to 60°.
  • Figs. 29 to 32 are representations of the relationships of ⁇ of the Euler angles ( ⁇ , 104°, 0°) with the acoustic velocity of boundary acoustic waves, the electromechanical coupling coefficient K 2 , the propagation loss, and the frequency temperature coefficient TCF, respectively, obtained in the same manner as Figs. 21 to 24.
  • Figs. 29 to 32 and crystalline symmetry show that when the Euler angle is in the ranges of -25° > ⁇ > 25° and 95° > ⁇ > 145°, the electromechanical coupling coefficient K 2 for P+SV boundary waves is advantageously reduced to 1% or less; and when the Euler angle ⁇ is in the ranges of -19° > ⁇ > 19° and 101° > ⁇ > 139°, the electromechanical coupling coefficient K 2 for P+SV boundary waves is more advantageously reduced to 0.5% or less.
  • electromechanical coupling coefficient K 2 for SH boundary waves is reduced to 1% or less at an Euler angle ⁇ in the range of 15° > ⁇ > 41° and is more advantageously reduced to 0.5% or less at an Euler angle ⁇ in the range of 21° > ⁇ > 38°.
  • Figs. 33 to 36 are representations of the relationships of ⁇ of the Euler angles (0°, 30°, ⁇ ) with the acoustic velocity of boundary acoustic waves, K 2 , the propagation loss, and the frequency temperature coefficient TCF, respectively, obtained in the same manner as Figs. 21 to 24,
  • an propagation angle ⁇ of 35° or more leads to an increased electromechanical coupling coefficient K 2 for SH boundary waves, consequently causing spurious responses.
  • the electromechanical coupling coefficient K 2 can be adjusted with the propagation angle ⁇ , as in the case in which SH boundary waves are used as the main mode.
  • Figs. 37 to 40 are representations of the relationships of ⁇ of the Euler angles ( ⁇ , 30°, 0°) with the acoustic velocity of boundary acoustic waves, the electromechanical coupling coefficient K 2 , the propagation loss, and the frequency temperature coefficient TCF, respectively, obtained in the same manner as Figs. 21 to 24.
  • Figs. 37 to 40 and crystalline symmetry show that when the Euler angle ⁇ is in the ranges of -32° > ⁇ > 32° and 88° > ⁇ > 152°, the electromechanical coupling coefficient K 2 for SH boundary waves is reduced to 1% or less; and when the Euler angle is in the ranges of -21° > ⁇ > 21° and 95° > ⁇ > 145°, the electromechanical coupling coefficient K 2 for SH boundary waves is advantageously reduced to 0.5% or less.
  • the IDT electrode may include first metal layers having a relatively high density and second metal layers having a relatively low density that are alternately stacked one on top of another.
  • An Al layer may be added to the stack of the metal layers, as long as the stack includes at least one layer of a metal having a density of 3000 to 21500 kg/m 3 .
  • Figs. 41 to 46 are representations of the measurement results of the resonance characteristics of the boundary acoustic wave device 10 according to the embodiment above.
  • Figs. 47 and 48 are representations of the measurement results of frequency temperature coefficient.
  • the IDT 14 had 60 pairs of electrode fingers, and the reflectors 15 and 16 had 51 electrode fingers.
  • the overlap of the IDT 14 was 30 ⁇ and the aperture width was 30.4 ⁇ .
  • the IDT 14 was weighted by varying the finger overlap such that the center overlap was 30 ⁇ and the overlap at both ends in the direction in which boundary acoustic waves propagate was 15 ⁇ .
  • the distance between the centers of electrode fingers between the IDT 14 and the reflectors 15 and 16 was 1.6 ⁇ m and the pitch of the electrode fingers of the IDT 14 and the reflectors 15 and 16 was 0.8 ⁇ m.
  • the electrode fingers had a line width of 0.4 ⁇ m, and the spaces between the electrode fingers were 0.4 ⁇ m in the direction in which boundary waves propagate.
  • Figs. 41 to 43 and 47 show the results of a structure including a SiN second medium 12
  • Figs. 44 to 46 and 48 show the results of a structure including a PSi second medium 12.
  • the resonant frequency temperature characteristics and the antiresonant frequency temperature characteristics are controlled by varying the thickness of the SiO 2 layer.
  • the absolute value of the frequency temperature coefficient TCF can be advantageously reduced by increasing the thickness of the SiO 2 layer.
  • the present invention is not limited to the boundary acoustic wave resonator having the electrode structure shown in Fig. 1, and may be applied to other boundary acoustic wave resonators having other electrode structures.
  • the invention is not also limited to resonators, and may be applied to a variety of filter devices using boundary acoustic waves, such as ladder-type filters, longitudinally coupled resonator filters, laterally coupled resonator filters, transversal filters, and boundary acoustic wave optical filters, and further applied to switching elements, such as boundary acoustic wave optical switches.
  • the material of the electrodes is not limited to Pt, Au, Ag, and Cu, and other metals having a higher density ⁇ than A1 can be used, such as Ni, Ti, Fe, W, and Ta.
  • the electrodes may include an additional thin layer made of a metal exhibiting high adhesion, such as Ti, Cr, NiCr, or Ni at the medium side to increase the adhesion to the mediums or the electric power resistance.
  • the electrodes, such as the IDT 14 may have a multilayer structure including metal layers, and such a thin adhesion layer may be disposed between the metal layers.
  • the IDT 14 is formed of a conductive material having a high density for a frequency band of over 1 GHz
  • the conductive material for the IDT may be formed to an excessively small thickness, and the electrode finger strips can have a high resistance.
  • the resistance of the electrode can be reduced by forming a multilayer structure of second medium/third medium/multilayer metal IDT/first medium including the multilayer IDT of a low-density conductive material and a high-density conductive material, such as Al and Au.
  • high-density metals help vibrational energy concentrate in the vicinity of the IDT. Accordingly, the electromechanical coupling coefficient can be increased and thus the same effects as in the above embodiment can be produced.
  • the thickness of the IDT may be adjusted to control the frequency by a variety of techniques, such as reverse sputtering, ion beam milling, reactive ion etching, and wet etching.
  • the thickness of the third medium may be further reduced by milling, etching or the like, or may be increased by deposition, such as sputtering or evaporation, and thus the frequency can be controlled.
  • the first to third media 11 to 13 may be formed of various types of materials including insulating materials and piezoelectric materials, such as Si, glass, SiO 2 , SiC, ZnO, Ta 2 O 5 , lead zirconate titanate ceramic, AlN, Al 2 N, Al 2 O 3 , LiTaO 3 , and KN, without being limited to the above-described materials.
  • insulating materials and piezoelectric materials such as Si, glass, SiO 2 , SiC, ZnO, Ta 2 O 5 , lead zirconate titanate ceramic, AlN, Al 2 N, Al 2 O 3 , LiTaO 3 , and KN, without being limited to the above-described materials.
  • the media may each have a multilayer structure composed of a plurality of media.
  • the electromechanical coupling coefficient K 2 for boundary acoustic waves can be advantageously increased.
  • a protective layer for preventing corrosive gases from permeating may be provided outside the first medium 11/IDT/piezoelectric material structure to enhance the strength of the boundary acoustic wave device.
  • the boundary acoustic wave device may be enclosed in a package in some cases.
  • the protective layer is not particularly limited, and can be made of a synthetic resin, such as polyimide resin or epoxy resin, an inorganic insulating material, such as titanium oxide, aluminium nitride or aluminium oxide, or a metal, such as Au, A1 or W.

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  • Physics & Mathematics (AREA)
  • Acoustics & Sound (AREA)
  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Surface Acoustic Wave Elements And Circuit Networks Thereof (AREA)
EP06713834A 2005-04-25 2006-02-16 Rand-schallwelleneinrichtung Withdrawn EP1879291A4 (de)

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PCT/JP2006/302693 WO2006114930A1 (ja) 2005-04-25 2006-02-16 弾性境界波装置

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US8049395B2 (en) 2006-12-25 2011-11-01 Murata Manufacturing Co., Ltd. Boundary acoustic wave device
US9941858B2 (en) * 2008-04-01 2018-04-10 Snaptrack, Inc. Electricoacoustic component with structured conductor and dielectric layer
US20140312736A1 (en) * 2008-04-01 2014-10-23 Epcos Ag Electricoacoustic Component with Structured Conductor and Dielectric Layer
CN102017407B (zh) * 2008-04-30 2014-03-19 株式会社村田制作所 弹性边界波装置
US8436510B2 (en) 2008-04-30 2013-05-07 Murata Manufacturing Co., Ltd. Boundary acoustic wave device
US20140117810A1 (en) * 2008-04-30 2014-05-01 Murata Manufacturing Co., Ltd. Boundary acoustic wave device
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CN102017407A (zh) * 2008-04-30 2011-04-13 株式会社村田制作所 弹性边界波装置
US8629598B2 (en) 2008-05-12 2014-01-14 Murata Manufactruing Co., Ltd. Boundary acoustic wave device
US8179017B2 (en) 2009-01-07 2012-05-15 Murata Manufacturing Co., Ltd. Boundary acoustic wave device having three-medium structure
US8264122B2 (en) 2009-01-26 2012-09-11 Murata Manufacturing Co., Ltd. Acoustic wave device
EP2211462A3 (de) * 2009-01-26 2013-11-13 Murata Manufacturing Co., Ltd. Akustikwellenvorrichtung
US8143762B2 (en) 2009-03-30 2012-03-27 Murata Manufacturing Co., Ltd. Elastic wave device using SH waves as the principal component

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US20090115287A1 (en) 2009-05-07
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JP4178328B2 (ja) 2008-11-12
JPWO2006114930A1 (ja) 2008-12-11

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